Oner reminds me that Aone is common for tops. Another trap for the oscitant xword tyro.
ONER has a long and noble history. No less an author than Charles Dickens (in The Old Curiosity Shop, 1840). "Miss Sally is sich a one-er for that." (Americans are often blamed fer that pronunciation (sich). Much of our bad pronunciation came from the good old British Isles. The rest from other furrin cuntries).
"You should see her eat. She is such a oner at eating." That came from Thackeray in 1862. Other examples are given; the word seems to have been fashionable in the 19th century.
Snuffy, nuttin' wrong with your usage. The OED has this example: 1869- "His figures run to anything between a fiver and two or three hundred oners." Pall Mall Gazette. (?? race track betting ??)
And then there is the boxing match which is a oner- Won with the first blow. 1861, but spelled onener.